High frequency dielectric measurement tool

Abstract

Disclosed dielectric logging tools and methods employ three or more receive horn antennas positioned between at least two transmit antennas, which can also be horn antennas. The logging tools can operate in the range between 100 MHz and 10 GHz to provide logs of formation permittivity, formation conductivity, standoff distance, and electrical properties of material in the standoff gap. Logs of water-saturated porosity and/or oil movability can be readily derived. The presence of additional receive antennas offers a significantly extended operating range, additional depths of investigation, increased measurement accuracy, and further offers compensation for tool standoff and mudcake effects. In both wireline and logging while drilling embodiments, at least some disclosed dielectric logging tools employ a set of three axially-spaced receive antennas positioned between pairs of axially-spaced transmit antennas. At least some disclosed methods employ absolute amplitude and phase measurements in response to alternate firings of the transmit antennas.

What is claimed is:  
     
       1. A dielectric logging tool that comprises:
 a first transmitter antenna; 
 a second transmitter antenna spaced axially from the first transmitter antenna; 
 three axially spaced receiver antennas positioned between the first and second transmitter antennas; and 
 a processing system, wherein each of the transmitter and receiver antennas are horn antennas providing a usable frequency range between about 100 Mhz and 10 GHz; and 
 wherein the processing system determines a standoff distance between the tool and a borehole wall based on measurements provided by each of the receiver antennas. 
 
     
     
       2. The tool of  claim 1 , wherein the horn antennas are tuned to operate at multiple signal frequencies. 
     
     
       3. The tool of  claim 1 , wherein the transmit antennas transmit signals having energy concentrated at one or more selected frequencies in the range between about 100 Mhz and 10 GHz, and wherein the tool further comprises electronics that measure amplitude and phase of the one or more selected frequencies in the receive signal. 
     
     
       4. The tool of  claim 1 , wherein the processing system further determines conductivity and permittivity of a formation penetrated by a borehole containing the tool. 
     
     
       5. The tool of  claim 4 , wherein the processing system further determines a conductivity and permittivity of a borehole fluid. 
     
     
       6. The tool of  claim 4 , wherein the transmitter and receiver antennas are positioned against a borehole wall during measurements. 
     
     
       7. The tool of  claim 1 , wherein the tool is a logging while drilling tool. 
     
     
       8. The tool of  claim 7 , further comprising a plurality of stabilizers, at least one of which maintains the antennas in close proximity to the borehole wall. 
     
     
       9. The tool of  claim 1 , wherein the tool is a wireline tool. 
     
     
       10. A logging method that comprises:
 moving a logging tool along a borehole, wherein the logging tool comprises at least three receive antennas positioned between at least two transmit antennas, wherein each of the transmitter and receiver antennas are horn antennas providing a usable frequency range between about 100 Mhz and 10 GHz; 
 measuring characteristics of a receive signal with each of the at least three receive antennas in response to operation of each of the at least two transmit antennas; 
 determining a standoff distance between the tool and a borehole wall based on measurements provided by each of the receive antennas; 
 determining formation permittivity values based at least in part on said characteristics; and 
 associating the permittivity values and standoff distance with tool position to provide a dielectric log. 
 
     
     
       11. The method of  claim 10 , wherein the logging tool is a wireline logging tool. 
     
     
       12. The method of  claim 10 , wherein the logging tool is a logging-while-drilling tool. 
     
     
       13. The method of  claim 10 , wherein the receive antennas are horn antennas that operate in contact with the borehole wall. 
     
     
       14. The method of  claim 10 , wherein the characteristics are indicative of receive signal attenuation and phase shift relative to a transmit signal. 
     
     
       15. The method of  claim 14 , further comprising determining formation conductivity values based at least in part on said characteristics. 
     
     
       16. The method of  claim 15 , further comprising determining electrical properties of material in the standoff gap. 
     
     
       17. A non-transitory information storage medium having software comprising:
 a measurement module that obtains measurements made via at least three receive antennas in response to two antennas spaced in opposite axial directions from the receive antennas, wherein the antennas are horn antennas providing a usable frequency range between about 100 Mhz and 10 GHz; 
 a processing module that determines a standoff distance between a logging tool and a borehole wall based on measurements provided by each of the receive antennas; 
 a tool module that predicts tool measurements based on estimated formation properties; 
 an inversion module that adjusts estimated formation properties until predictions of the tool module match the measurements obtained by the measurement module; and 
 a display module that provides a log of at least one formation property as a function of tool position or depth. 
 
     
     
       18. The medium of  claim 17 , wherein the at least one formation property comprises permittivity. 
     
     
       19. The medium of  claim 17 , wherein the at least one formation property comprises water-saturated porosity. 
     
     
       20. The medium of  claim 17 , wherein the at least one formation property comprises oil movability. 
     
     
       21. The medium of  claim 17 , wherein the measurements include attenuation and phase shift relative to the transmit signal.